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charset=us-ascii Content-Disposition: inline In-Reply-To: <20230313083452.1319968-1-yosryahmed@google.com> X-Stat-Signature: jyrw4osudub3widmksjx6asysq8k53aj X-Rspam-User: X-Rspamd-Queue-Id: C045C140023 X-Rspamd-Server: rspam06 X-HE-Tag: 1678788052-594161 X-HE-Meta: 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 Bm+Ee527 hA49D2nf6r1QlakAOun2zzNNMf97mIl4dSH6A7DeYc1y2S5i1d6eG7Rr4AuwJyrY3CqkfjKYtLCjNhLclOAhH8R05pdFto+G6x1773f5DcbMXFFEYRIe7TtaE/yb3vrZrWKCFP02pNESn2wv8TF5XyqMWGHXc3Synh7HFL2BpEaQUG2peMLmMiDhnUOo2yQG8NlptPKqatioPYNJBfIAw2pU0j4w8eWX6xssjS1p4KZk7Q2YmZJqjVIzKcG1ROEBGmXXrQlwYDVYLtbbNnw2sDmoV4yhzkVBZ7r4R8bK6OTnau9uOL9biKaL86pj0hMCd+q3pj3zleNAMxXFr5qKArU6KOcBLImbTItEagnWXXM0zO5c= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon 13-03-23 08:34:52, Yosry Ahmed wrote: > From: Hugh Dickins > > In a kernel with added WARN_ON_ONCE(PageTail) in page_memcg_check(), we > observed a warning from page_cgroup_ino() when reading > /proc/kpagecgroup. This warning was added to catch fragile reads of > a page memcg. Make page_cgroup_ino() get memcg from compound_head(page): > that gives it the correct memcg for each subpage of a compound page, > so is the right fix. > > I dithered between the right fix and the safer "fix": it's unlikely but > conceivable that some userspace has learnt that /proc/kpagecgroup gives > no memcg on tail pages, and compensates for that in some (racy) way: so > continuing to give no memcg on tails, without warning, might be safer. > > But hwpoison_filter_task(), the only other user of page_cgroup_ino(), > persuaded me. It looks as if it currently leaves out tail pages of the > selected memcg, by mistake: whereas hwpoison_inject() uses compound_head() > and expects the tails to be included. So hwpoison testing coverage has > probably been restricted by the wrong output from page_cgroup_ino() (if > that memcg filter is used at all): in the short term, it might be safer > not to enable wider coverage there, but long term we would regret that. > > Signed-off-by: Hugh Dickins > Signed-off-by: Yosry Ahmed Acked-by: Michal Hocko Thanks! > --- > > (Yosry: Alternatively, we could modify page_memcg_check() to do > page_folio() like its sibling page_memcg(), as page_cgroup_ino() is the > only remaining caller other than print_page_owner_memcg(); and it already > excludes pages that have page->memcg_data = 0) > > --- > mm/memcontrol.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 5abffe6f8389..e3a55295725e 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -395,7 +395,7 @@ ino_t page_cgroup_ino(struct page *page) > unsigned long ino = 0; > > rcu_read_lock(); > - memcg = page_memcg_check(page); > + memcg = page_memcg_check(compound_head(page)); > > while (memcg && !(memcg->css.flags & CSS_ONLINE)) > memcg = parent_mem_cgroup(memcg); > -- > 2.40.0.rc1.284.g88254d51c5-goog -- Michal Hocko SUSE Labs